Currently, the reason Runaan's is probably ignored on most adc's is BECAUSE it doesn't give crit. ADC's are mostly entrusted with melting single targets one at a time, with a few notable exceptions (kalista) but still, only a few exceptions. When you go Runaan's, you're basically saying "I could have bought phantom dancer or shiv and be critting this one dudes face off, but decided to get runaan's instead and triple melt with bork." With these changes, you're not necessarily picking one choice over the other when you get runaan's.
Second problem this targets on Runaan's: You can quickly cap your attack speed on certain adc's with it, or come so close as to make AS steroids meaningless. Replacing the attack speed with crit and move speed is useful.
Now let's look at the numbers if you really think it's a nerf:
Before: over 2 bolts and main target, you get 200% of ad in damage.
After: Now with a lot of adc's able to get 100% crit almost certainly (gg yasuo/trynd standouts) it's 225%. Not to mention, one out of every 3 crits on the main target are critting due to this item now, which is an additional 150% every third attack with IE, or 50% on average. So it's basically 275%. Oh but it's at a slower attack speed rate... unless you have huge AS capping buff like tristana anyway, or, heck, you're almost any other ADC in the game.
Before: Helped melt multiple targets and get more lifesteal
After: Still melts multiple targets with slightly MORE damage, and, for most adc's, who will STILL cap attack speed, doesn't nerf actual attack speed at all, and, well, deletes the main target MORE quickly than if you DIDN'T buy it, unlike before.
You have to look at how this item stands in a full build, not just by itself when analyzing the changes. ADC's without AS steroids might not do so well with it, but many ADC's I can see becoming major issues with these changes. It aint a nerf, not by a longshot.